The recording density in magnetic recording is improving at an annual rate of 100% since an advent of a GMR head using the giant magneto-resistance effect (GMR effect). A GMR element includes stacked films having a sandwiched structure of ferromagnetic layer/nonmagnetic layer/ferromagnetic layer. The GMR element is an element using the magneto-resistance effect of a spin valve film for applying an exchange bias to one ferromagnetic layer for fixing magnetization thereof and changing the magnetization direction of the other ferromagnetic layer by an external magnetic field for detecting change in the relative angle of the magnetization directions of the two ferromagnetic layers as change in the resistance value. A CIP (Current In Plane)-GMR element for allowing an electric current to flow into the film surface of a spin valve film and detecting resistance change and a CPP (Current Perpendicular to Plane)-GMR element for allowing an electric current to flow perpendicularly to the film surface of a spin valve film and detecting resistance change are developed. The magnetic resistance ratio (MR ratio) of each of the CIP-GMR element and the CPP-GMR element is about several percent, and it is considered that the elements will be able to cover the recording density of up to about 200 gigabits/inch2.
To cover higher-density magnetic recording, development of a TMR element using the tunneling magneto-resistance effect (TMR effect) is pursued. The TMR element includes stacked films of tunnel insulating layer made up of ferromagnetic layer/insulator/ferromagnetic layer, and a voltage is applied to the nip between the ferromagnetic layers for allowing a tunnel current to flow. The TMR element is an element for using the fact that the magnitude of the tunnel current changes depending on the direction of magnetization of the top and bottom ferromagnetic layers and detecting change in the relative angle of magnetization as change in the tunnel resistance value. The TMR element has maximum MR ratio of about 50%. Since the TMR element has a larger MR ratio than the GMR element, the signal voltage also becomes large.
However, in the TMR element, not only the pure signal component, but also the noise component of shot noise becomes large. Accordingly, the S/N ratio (signal-to-noise ratio) needs to be improved in the TMR element. The shot noise is caused by fluctuations of current occurring as electrons irregularly pass through a tunnel barrier, and increases in proportion to the square root of the tunnel resistance value. Therefore, in order to suppress the shot noise and obtain a necessary signal voltage, the tunnel insulating layer needs to be thinned for lessening the tunnel resistance.
As the recording density becomes higher, it becomes more necessary to lessen the element size to a size of the same degree as a record bit. Thus, the density becomes higher, it becomes more necessary to lessen the joint resistance of the tunnel insulating layer, namely, thin the insulating layer. At the recording density of 300 gigabits/inch2, joint resistance of 1Ω·cm2 or less is required and a tunnel insulating layer having a thickness of two layers of atoms in terms of the film thickness of an Al—O (aluminum oxide film) tunnel insulating layer must be formed. As the tunnel insulating layer is made thinner, a short circuit between upper and lower electrodes occurs more easily and degradation of the MR ratio is incurred and therefore it becomes dramatically difficult to manufacture elements. Thus, it is assumed that the limit of the recording density of the TMR element may be 300 gigabits/inch2.
Each of the elements described above uses the magneto-resistance effect in a wide sense; a problem of magnetic white noise common to the elements has sprung up in recent years. Unlike electric noise such as shot noise described above, the magnetic white noise is caused by thermal fluctuations of magnetization and thus becomes more dominant with miniaturization of the elements. Therefore, it is considered that an element covering a higher density will outstrip the electric noise. In order to avoid the magnetic white noise and further increase the recording density of magnetic recording, a minute magnetic sensor operating according to a principle different from the former magneto-resistance effect becomes necessary and development of a magnetic oscillation element as such a magnetic sensor is pursued.
In the highly integrated LSI field, metal wiring of Al, Cu, etc., is used at present; a problem of a signal delay caused by resistance or inductance with an increase in the wiring length becomes obvious. To solve the signal delay problem and realize an LSI that can operate at higher speed, development of a wireless LSI for transferring signals using minute magnetic oscillation elements and minute magnetic reception elements without using wiring is also pursued.
Further, development of a communication device such as a spin-torque diode using the singular nature of the magnetic oscillation element is also started.
An example of such device is described in the following document.
R. Sato, et al. J. Magn. Magn. Mat. vol. 279, p. 36 (2004)
It is expected that the magnetic oscillation element will be applied to various fields including magnetic recording as described above; however, the magnetic oscillation element has disadvantages in that thermal noise accompanying thermal fluctuations of magnetization is large and the signal purity is low because the magnetic oscillation element is a nano-scale minute element.